Struggling to find focus especially at higher mgnifications is the classic sign of a cheap 'spherical' mirror. Always look for the specs to quote a 'parabolic' mirror if not, don't buy.

I've also heard of some cheaper scopes using a piece of normal (back silvered) mirror for the diagonal instead of a proper optical (front-polished) mirror.

For good quality, at a fair price, stick with SkyWatcher, Meade, Celestron, TAL, Revelation, Konus.

Avoid Tasco, Optisan, Optron, Seben, Event Horizon, Bausch & Lomb, Bushnell, and generally anything that looks too good to be true.

Also watch out for cheap Celestrons and Meades, sold from a non-specialist dealer (places like Argos, Jessops etc) they licensed out their name to some cheap far eastern manufacturers. If you see one, check their website if the model isn't on there, it's not a proper Celestron/Meade.

Bresser are a funny one their refractors and binoculars are generally OK, but avoid anything with a mirror in it. Same goes for 'Phenix' It's a sad state of affairs, but the optics market is an absolute minefield to the uninitiated.

Bonesetter You will not be disappointed with a SkyWatcher Explorer I've had the 6" model for well over a year now it's excellent the 8" should be even better.

If you've never used a scope like this, be prepared it is HUGE!!!!!